How to attach eye armature to rigify armature?
Rigify armature is really confusing with all those names so what I always do to parent head with new bones for the eyes is to select head bone in edit mode, add a new bone and then add other bone that is going to move one eye and then do the same for the other eye.

After that select the bone that is in the middle connecting eyes with head and delete it. You will have eyes parented with the correct bone head.

I know there are probably other way to do that